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First, you should add the Earth Tube Standard Density Volume Flow Rate output variable to confirm that the supply airflow is constant as you change the earth tube radius. Once you have confirmed that, you can use the following output variables for further debugging of why you see changes in supply air temperature:

• Earth Tube Air Mass Flow Rate (would tell you how air temperature change is affecting air density)
• Earth Tube Ground Interface Temperature
• Earth Tube Outdoor Air Heat Transfer Rate

It doesn't look like there is an output variable for air velocity, so you will need to manually calculate that with volume flow rate and pipe area based on radius. This can either be post-processed after each simulation, or you can use Energy Management System (EMS) (Scripting module in DesignBuilder) to create a custom air velocity output created by EnergyPlus.